IN NATURAL PHILOSOPHY. 45 



m turning to vapor renders latent over 900 of heat. This is, 

 of course, entirely lost to the consumer. 



45. Why does not green wood snap ? 



The pores are filled with water instead of air. The water 

 does not expand rapidly enough to burst off the coverings of 

 the cells, and so simply oozes out gradually and is vaporized. 



46. Why will a piece of metal dropped into a glass or porce- 

 lain dish of boiling water facilitate the ebullition ? 



The rougher surface of the metal aids in the formation and 

 disentanglement of the steam-bubbles. The bubbles cling 

 longer to a smooth than to a rough surface. This is one 

 cause of that bumping sound often noticed when liquids are 

 boiling in glass dishes. 



47. Which can be ignited the more easily with a burning- 

 glass, black or white paper f 



Black paper, since it is a much better absorber of heat. 



48. Why does the air feel colder on a windy day ? 

 Because fresh portions of cold air are brought constantly in 



contact with our bodies. 



49. In what did the miracle of Gideon 's fleece consist ? 

 The hard threshing-floor was a better conductor of heat 



than the porous fleece ; hence, naturally, the dew would 

 collect on the latter more readily than en the former. In 

 the miracle, the conditions remaining the same, the results 

 in the two cases were reversed. (Judges, vi. 37-40.) 



50. Could a burning-lens be made of ice ? 



Burning-lenses have been made of that material. The rays 

 have no heating power until the waves of ether are stopped. 

 They do not elevate the temperature of the medium through 

 which they pass. 



5 1 . Why is an iceberg frequently enveloped by a fog ? 

 The moisture of the air is condensed upon its cold surface. 



52. Would dew gather more freely on a rusty stove than on 

 a bright kettle? 



It would, because the rusty iron surface is a good radiator. 



53. Why is a clear night colder than a cloudy one f 



